A spark plug is known from European Patent No. 0 101 547 and is designed for use in a gasoline engine.
The known spark plug includes an essentially tube-shaped, metallic shell which has a thread for mounting into the cylinder head of the engine, as well as a hexagonal surface as a means for assembly into the engine. The shell has a through bore in which an essentially rotationally symmetric electrical insulator is situated, a center electrode being fixedly attached to it; the center electrode cooperates with a ground electrode which is situated at the end section of the shell on the combustion chamber side.
In the assembled state, the end section of the shell, i.e., the spark plug, protrudes into the combustion chamber on the combustion chamber side.
In practice it is important to design internal combustion engines so that their fuel consumption and emission levels are low. In this connection it is also important to cool the engine in order to achieve high thermal efficiency, which takes place via cooling ducts. In order to dimension the cooling ducts sufficiently generously, the spark plugs installed in the cylinder head of the engine are only to be allowed a limited space.
However, this is not achievable using contemporary spark plugs without the mechanical and electrical characteristics of the spark plugs being changed in an adverse manner.
The spark plug according to the present invention has the advantage over the related art that it is optimized with regard to the space required for installation, so that the space required for installing the spark plug is small, and larger cooling ducts, exhaust ports, and intake ports in the cylinder head may be implemented, in particular in the region of the sealing seat compared to the related art. This makes possible a thermodynamically favorable combustion with simultaneously high performance of the engine, since the cylinder head may be provided with comparatively large cooling ducts and large exhaust valves and intake valves.
In order to lower the operating temperature and the natural vibration of the ground electrode compared to conventional spark plugs, a spark plug having length/diameter ratios according to the present invention is provided with a very long thread, i.e., a type of thread extension which is an integral part of the threaded shank. As a rule, the thread is an integral part of a plug shell which accommodates what is known as a plug stopper.
The dimensions of the spark plug according to the present invention are such that, compared to a spark plug according to the related art, this spark plug, despite the reduced assembly space achieved, meets all requirements of a modern spark plug, including, for example, adequate breakdown resistance and twist-off resistance.
A projected spark position reaching well into the combustion chamber of the engine may also be implemented in a spark plug according to the present invention, without the danger of a vibration breakdown of the ground electrode and without the temperatures affecting the spark plug reaching a value above the permissible level for the material used. The spark plug is therefore suitable in particular for application in fuel injector systems.
The spark plug according to the present invention may in particular be designed such that the ratio of the sum of the length of the threaded shank and the length of the assembly shank to the sum of the diameter of the assembly shank and the diameter of the thread for a thread diameter of 12 mm and a length of the threaded shank of between 19 mm and 22 mm assumes a value between 1.08 and 1.20, or it assumes a value between 1.2 and 1.50 for a threaded shank length of between 25 mm and 29 mm, or a value of between 0.94 and 1.05 for a threaded shank length of between 17.5 mm and 22 mm.
The spark plug according to the present invention advantageously has a conical sealing seat in order to achieve accurate positioning of the ground electrode in the combustion chamber of the engine. The conical sealing seat is preferably positioned in the transition area between the threaded shank and the assembly shank.
An assembly shank is considered optimized in terms of assembly space, for example, when the ratio between the assembly shank diameter and the assembly shank length is less than 1.30, or greater than 1.70, i.e., for example 1.26 or 1.72.
The spark plug according to the present invention is advantageously provided with a marking in order to achieve an accurate alignment of the ground electrode in the combustion chamber with small tolerances. The marking, for example, may be a mark on the double hexagon used as assembly means. The mark may be a marking on a connecting nut or a connecting bolt, such as a bore, an indentation, or a color speck; but it may also be a colored marking on the electrical insulator.
Furthermore, it is advantageous for optimal mounting of the plug stopper of the spark plug in the spark plug shell if a compressed powder pack is situated in an annular gap between the plug stopper and the plug shell. In order to produce the powder pack, using talcum for example, the appropriate powder is poured into the gap between the plug stopper and the plug shell during assembly of the spark plug and compressed. During final assembly of the spark plug, a rim of the plug shell is deformed radially in the direction of the plug stopper. The rim of the shell compresses the powder pack, whereby the stopper is mounted in the shell.
In order to optimize the powder pack compression, the powder pack may, at least on one side in the axial direction of the spark plug stopper, be limited by a pressure ring. In this case, during final assembly of the spark plug, the rim is also deformed radially in the direction of the plug stopper, so that the powder pack is compressed by both the rim and the ring. By compressing the powder pack in this way, the plug stopper is mounted in the plug shell, ensuring a high degree of heat insulation.
A particularly good powder pack compression and the associated heat insulation of the spark plug may be achieved if the powder pack is limited by a pressure ring on both sides.